1. Field of the Invention (Technical Field):
The invention relates to an apparatus and method for performing gas purging during metallurgical welding, particularly for purging the interior of industrial process piping systems and pipelines during welded assembly.
2. Background Art:
In fabricating welded joints in alloy piping, it is extremely desirable to exclude reactive gases from the heated weld area in order to minimize detrimental metallurgical and chemical alterations in the metal alloy. Welds performed in the presence of reactive gases tend to cause metallurgic transformations which adversely affect the strength and corrosion resistance of many alloy materials. For this reason, various efforts have been made to devise systems and methods for purging reactive gases, such as oxygen and nitrogen, from the weld area during fabrication.
The need for gas purging extends to the welded fabrication of alloy steel pipes. The standard techniques for excluding reactive gases from the region of the weld on the exterior of the pipe are the use of Gas-Tungsten Arc Welding (GTAW) or the use of Gas-Metal Arc Welding (GMAW). With these methods, the arc and the immediate vicinity of the progressing weldment on the pipe exterior are purged of reactive gases by a blast of inert gas, such as argon, from an outlet on the welder's torch. To protect the interior side of the weldment, a section of the pipe adjacent to the weld is blocked off with purge dams, one dam each on either side of the weld. The blocked-off section of the pipe is filled with inert gas from a single injection source, to purge the interior of the pipe of reactive gases.
In the current practice, it is typical to admit argon into the enclosed section of pipe through one of the purge dams and vent it through the joint to be welded and/or a vent in one of the purge dams. Since the gap between the two pieces of pipe to be joined initially presents a significantly large area through which admitted argon can escape, the entire circumference of the gap save a small hole is temporarily covered with tape while the interior of the pipe is purged of air. Subsequent to a determination that the oxygen concentration within the interior of the pipe is within acceptable limits, the welder opens up a segment of the tape (usually one fourth the pipe circumference or less) and begins to apply the first pass of weld metal (the "root bead").
In the existing art, the argon purge flow rate usually is uncontrolled or poorly controlled. Frequently, the only purge gas discharge control is the simple expedient of employing a pressure regulator on the inert gas source. Such rudimentary control presents serious problems, for if the gap being closed is wide enough, removing tape to open any arcuate segment of the gap can result in reactive air being drawn into the pipe interior. On the other hand, inert gas pressures required to maintain an outward flow through the gap from inside the pipe are likely to be high enough to deform the molten weld metal, especially when closure is approached near the completion of the circumferential weld. Such deformations, which typically are discovered only after X-ray testing, unacceptably impact the quality of the weld.
U.S. Pat. No. 2,819,517 to Pursell, entitled Method of Welding Pipe Ends Together, discloses the use of an exterior pressure regulator at the source of the purge gas, purportedly to avoid over-pressurizing the purged chamber in the pipe. No means for reducing pressures to avoid weldment distortion during closure are disclosed, and there is no teaching of inflatable sealing rings or specialized directional flows.
U.S. Pat. No. 2,802,092 to Hauck, entitled Method and Apparatus for Welding, shows a purge dam apparatus and method requiring that a survey hole be drilled through the pipe wall, through which purge gas is introduced. Survey holes such as those taught by Hauck are banned by modern codes; no pressure sensors or specialized directional flows are disclosed.
U.S. Pat. No. 4,916,281 to Flasche et al., entitled Gas Back-Purging During Welding of Pipe, teaches control of a gas back purging process to maintain a constant gas flow and a constant ratio of torch "shielding" gas to purge "backing" gas. No specialized bidirectional flows are suggested.
Soviet Union Patent No. SU 1323-318-A to Volodkin discloses a weld protection device provided with a pressure sensor to assure constant gas pressure. Japanese Patent No. JA 0215375 shows a "roll-out" welder for use in the workshop or factory environment. The flow of inert gas is controlled according to the pressure of the gas in a closed space. A single valve is utilized, which may result in unacceptably large pressure and discharge surges during operation. No bidirectional flow is indicated.
An article found on page 57 of the August, 1989, issue of Process Engineering magazine contains a succinct discussion of various reasons and modes known in the art for performing gas purging during piping systems welding.
Nevertheless, a need remains for a method and apparatus for maintaining an inert gas pressure inside the pipe and an inert gas flow rate which are adequate to prevent incursion of reactive gases at any stage of weldment completion, yet low enough to prevent distortion of the molten weld metal as completion is approached, and which are so directed as to give uniform, high-quality welds throughout the length of the abutment circumference. Likewise, there is a need for a more efficient and orderly delivery of inert gas to the vicinity of the weld, as opposed to a single point source injection into the enclosed pipe interior.